Oxidation distillation method using the volatility of ruthenium tetroxide (RuO4) is known as a method for separating and recovering ruthenium. For example, Journal of the Atomic Energy Society of Japan (Vol. 28, pp. 493-500) discloses a method for recovering ruthenium tetroxide by adding an oxidizer to a residue containing platinum group metal, dissolving it by alkali fusion, and flowing chlorine through the solution, and a method for recovering ruthenium tetroxide into a caustic soda solution by dissolving a ruthenium-containing precipitate in nitric acid, adding potassium permanganate to the solution, and blowing air into the solution. In these methods, a gas-induction tube is inserted into a flask heated by a heating mantle, and gas is introduced into it.
Japanese Patent Application Laid-Open Publication No. 2004-332041 discloses a method for separating and recovering ruthenium by distilling ruthenium tetroxide from a solution containing platinum group metal using sodium bromate (NaBrO3), etc. as an oxidizer.
Japanese Patent Application Laid-Open Publication No. 2004-353480 relates to a method for recovering ruthenium by introducing ruthenium tetroxide vapor into a hydrochloric acid solution and converting it into ruthenium chloride. However, this method suffered from a problem that when air is blown into the solution to enhance efficiency of the recovery, harmful ruthenium tetroxide vapor might leak out from a connection, and the connection might be disengaged or blown out due to the increased pressure when a possible clogging occurs. Therefore, this publication discloses a method in which the pressure within the apparatus is reduced by a suction pump, and ruthenium tetroxide is transferred from a reaction tank into the hydrochloric acid solution along with the sucked air. The publication also states that the range of pH must be 0.5-2.5 for the conversion into ruthenium tetroxide by sodium bromate, and the oxidizer with sufficient concentration is necessary to efficiently convert ruthenium into ruthenium tetroxide before pH is raised by the decomposition of the oxidizer.
There is a problem that ruthenium tetroxide, which has large specific gravity, tends to accumulate and remain in the bottom of the reaction tank when ruthenium tetroxide is transferred from the reaction tank into the hydrochloric acid solution along with the introduced air under reduced pressure created by the suction pump. If ruthenium is remained in the reaction tank, it hinders the recovery of platinum group metal other than ruthenium from the solution.
Therefore, stirring of the solution within the reaction tank was first examined. As it was not suitable to use resin or metal as a material for the apparatus since ruthenium tetroxide is highly oxidative, the material was limited to glass, quartz or the like. However, it was found out that stirring under reduced pressure while keeping a good sealing property was difficult when these materials were used. Consequently, it was necessary to flow air for a long time to remove ruthenium tetroxide from the reaction tank.